In recent years, electronic devices with liquid crystal displays, organic EL displays, etc. and electronic devices with touch panel displays have been widespread. Since glass materials have high surface hardness, they are widely used as materials of cover glasses of displays of these electronic devices. Cover glasses of displays are sometimes chemically strengthened to improve their mechanical strength.
Chemical strengthening is a technique of replacing alkali metal ions contained in the glass surface by monovalent cations having a larger ionic radius so as to form a compressive stress layer on the glass surface. Chemical strengthening is often performed by replacing lithium ions (Li+) by sodium ions (Na+) or by replacing sodium ions by potassium ions (K−).
A glass composition suitable for chemical strengthening disclosed in Patent Literature 1 contains 64 to 68 mol % SiO2, 12 to 16 mol % Na2O, and 8 to 12 mol % Al2O3. In this glass composition, the content of Na2O is higher than that of Al2O3 by 2 to 6 mol %, and the total content of alkaline earth metal oxides (MgO+CaO+SrO) is adjusted to 5 to 8 mol % (claim 1). In addition, the glass composition described in Patent Literature 1 has a melting temperature of less than 1650° C. and a liquidus viscosity of at least 13 kPa·s to be adapted to a down-draw process. In the glass compositions described as examples in Patent Literature 1, the contents of Al2O3 and Na2O are 13.99% or more and 13.76% or less, respectively, in terms of mass %. Furthermore, in the examples of Patent Literature 1, the temperatures at which the glass compositions have a viscosity of 200 P (200 dPa·s) are 1536° C. or more and the temperatures at which the glass compositions have a viscosity of 35 kPa (35000 dPa·s) are 1058° C. or more.
A strengthened glass substrate suitable for use in a touch panel display disclosed in Patent Literature 2 contains, in mass %, 45 to 75% SiO2, 1 to 30% Al2O3, 0 to 20% Na2O, and 0 to 20% K2O (claim 3). Glass substrates disclosed as examples in Patent Literature 2 contain 13.0 to 24.0 mass % Al2O3 and 4.1 to 14.5 mass % Na2O. Furthermore, in the examples, the temperatures at which the glass substrates have a viscosity of 104 dPa·s are 1122° C.
A working temperature and a melting temperature are known measures of the high-temperature viscosity of glass. In a float process, the working temperature is a temperature at which glass has a viscosity of 104 dPa·s, and hereinafter may be referred to as T4. The melting temperature is a temperature at which glass has a viscosity of 102 dPa·s, and hereinafter may be referred to as T2.